Filter materials are well known and widely used as filter paper, for example, in making tea bags, coffee bags, coffee filters and the like. Conventional filter materials consist of one layer of natural fibers which extends in a substantially planar manner and is covered by a substantially planar heat-sealing plastic layer. These filter materials are manufactured by forming, in a first run on a paper-making machine, a base layer predominantly consisting of natural fibers. Thereafter, the material is partially dehydrated using a vacuum chamber. Next, a second layer consisting of heat-sealing plastic fibers is deposited on the base layer. The second layer is then dehydrated through the base layer resulting in the formation of essentially straight ducts or pores which cross between both layers.
Products made from the above-described filter materials, such as tea or coffee bags, are intended to retain a maximum amount of particles, for example tea or coffee particles, while allowing for the rapid diffusion of hot water through the filter material in order to achieve an aromatic, well-colored beverage in a short time. Accordingly, the filter material must be commensurately porous. Previously, a trade-off had to be obtained between a filter material with minimal particle transmission and a filter material capable of high liquid diffusion. The straight pores extending through the filter material as described above does insure high water or other liquid diffusion, but such material is disadvantageous with regard to particle transmission, especially in making tea or coffee bags, since the straight cross-pores permit passage of a correspondingly large quantity of particles through the filter material.